With the advent of advanced ducted propulsors which utilize a variable pitch fan in the gas turbine engine, there has been a need for an effective, lighter and less expensive pitch change mechanism that will vary the pitch of the fan. It is well known that because the pitch change mechanism is mounted in the fan hub it must not only be light weight it also must be compact. To this end several concepts have been disclosed that attempt to serve this purpose and they include hydro-mechanical and electomechanical versions that incorporate either a ball screw and ball screw nut pitch change actuator or a hydraulic actuator.
U.S. Pat. No. 5,199,850 granted to E. H. Kusiak and P. A. Carvalho on Apr. 6, 1993 entitled "Pitch Stop Assembly For Variable Pitch Change Propulsor" and assigned to United Technology Corporation, the assignee common with this patent application discloses a mechanical pitch actuation system that is capable of being utilized in a ducted fan for gas turbine engines, sometimes referred to as Advanced Ducted Propulsor (ADP). As is disclosed in this patent, a well known ball screw actuator is utilized to effect pitch change movement. The ball screw actuator consists of a ball screw that during fixed pitch rotates at the same angular velocity as the fan. By virtue of a differential gear train operatively connected to the ball screw, an input signal to the differential gear train changes the rotational speed relationship of the differential gear train to the ball screw to effectuate a rotation of the ball screw to either a clockwise or counter clockwise direction for pitch change movement. The ball screw nut operatively connected to the ball screw is caused to translate. This rectilinear movement is converted to rotary movement of the fan blades by the trunnion eccentrically attached to the base of the fan blade. The motion from the ball screw nut to the trunnion is transmitted by a yoke and linkages to rotate the plurality of blades in unison about their longitudinal axis. Obviously, the input signal to the differential gear train will ultimately rotate the blades to either a course or fine pitch.
U.S. Pat. Nos. 5,183,387 and 5,205,712 granted to Huggert et al on Feb. 2, 1993 entitled "Fault-Tolerant Apparatus For Controlling Blade Pitch" and granted to Hamilton on Apr. 27, 1993 and entitled "Variable Pitch Fan Gas Turbine Engine", respectively, for example, disclose electrical pitch change mechanism. These patents disclose electrical induction machines with fixed windings cooperating with rotating windings fixed to the rotating fan shaft that are in a fixed pitch position until the electrical pitch change mechanism is actuated. Pitch change is effected by exciting the fixed winding to induce a braking effect of the rotating connecting mechanism that is translated to the fan blades through suitable linkages. Typically two or three induction machines are utilized to vary the pitch to course, fine blade angles and feather.
One of the systems contemplated for the ADP propeller pitch change mechanism is a dual electrical induction machine system that is operatively connected to the ball screw of the ball screw actuator similar to the one disclosed in U.S. Pat. No. 5,199,850. In this system the ball screw is rotated by a gear train which results in translating motion of the ball screw nut. This translating motion is converted into rotary motion at the blades through trunnions attached to the base of each blade.
To change pitch, two electrical induction machines are utilized; one for increase pitch and the other for decrease pitch. Each induction machine includes a stator winding and a squirrel cage rotor. The rotor winding is mounted around the engine output shaft and by energizing the stator winding the magnetic field in the gap between the winding varies to impart a braking effect on the rotor. The rotor of each induction machine drives a ring gear that is operatively connected to the feed drive that drives the ball screw. One of the rotors is directly connected thereto while the other imposes a gear train between the rotor output shaft and the ball screw so as to impart rotation in a direction that is opposite to the direction of the other induction machine. Hence, one of the induction machines serves to position the blades toward fine pitch change and the other serves to position the blades toward course pitch change. The rotors of both induction machines are mounted on the engine output shaft connected to the ball screw gear train with a feed through gear. By energizing one of the windings of either induction machines, the angular velocity of that rotor excited by that winding becomes retarded relative to the engine shaft to drive the rotary ball shaft and in turn translate the ball screw nut. The trunnion eccentrically mounted to the base of the blade coupled to the translating ball screw nut converts the axial motion to rotary motion for varying the pitch of the blade.
It has been found that this system has certain limitations which is predicated by the necessary gear ratio between the rotors and blade pitch change axis in combination with the maximum torque loading conditions. The maximum gear ratio is sized by the minimum fan speed (30%) plus the maximum beta rate (reversing-30.degree./sec). Under this condition a minimum slip speed between the inductive machine rotors and the stationary coils must be maintained for the machines to produce the required torque. The condition that determines the torque requirement of the inductive machines is the 100% speed case which requires only half the beta rate. This system is also limited because pitch change can only be effectuated when the fan is rotating. Hence, in this system, an additional inductive machine with motoring capabilities is required for static conditions.
We have found that we can obtain a less complex, lighter weight and smaller envelope system which requires fewer components by incorporating a DC inductive brake and a bidirectional AC inductive motor uniquely used in combination to optimize gear ratio which minimizes size and weight of the inductive machines. The two inductive machines are connected to a single rotor assembly. The rotor assembly, in turn, is connected to the ballscrew gear train via a single feed through gear. This design eliminates the need for a second rotor assembly, eliminates one induction machine required for static conditions, eliminates the reversing idler gear and reduces the torque requirements of the remaining inductive machines by allowing an increase in the gear ratio to the blades.
According to this invention, the inductive brake would be used in the increase pitch direction at the maximum load case (100%). For increase pitch at the lower fan RPM (30%) and the higher beta rates (30.degree./sec.) a combination of brake operation at high slip speed and motor operation at low slip speed would be used. This mode of operation allows a higher gear ratio between the inductive machines and the blade pitch change axis, reducing the torque requirements at the induction machine. The motor can be used in a range of slip speeds that extend for positive slip speed through to zero slip speed and to negative slip speeds. Obviously, the reduction of the torque requirements greatly reduces the weight and envelope of the inductive machines. As used herein and understood in this technology, slip speed is the relative rotation between the stator and rotor of the inductive machine.
Additionally, instead of utilizing an independent inductive machine for feather as is the case in the heretofore known design the inductive motor is utilized for both pitch change toward decrease pitch and the independent feather function.